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This is the first part of a series of posts that gives a brief and selective historical look at research on sex differences in the brain. In this post, I will focus on early research and how it molded my research interests.
Although sex differences in cognitive abilities have been reported for a long time, theorists in the early 1970s relied heavily on the reviews published by Broverman, Klaiber, Kobayashi, and Vogel (1968), and Garai and Scheinfeld (1968) to conclude that sex differences were found in two areas: spatial (favoring males) and verbal abilities (favoring females). From the very beginning in this area of research, proposed explanatory factors have been numerous, but they can be put into either the social/environmental or the biological side. It should be clear that my emphasis on sex differences in the brain here means that I will only focus on biological causes. In fact, it is rather obvious to think that cognitive sex differences are most likely due to sex differences in the brain as all behaviors are caused by the brain! However, the first approach I came across made use of the (at the time) recent discovery that we could examine the functions of each cerebral hemisphere by means of perceptual asymmetries. Specifically, it is part of common knowledge nowadays that the left cerebral hemisphere handles verbal functions, whereas the right hemisphere handles nonverbal functions. Of course, with the techniques in use now, this distinction, known as “laterality” simplifies matters, but we will discuss that later.
Perceptual asymmetries reflect laterality as a result of properties of, for example, the auditory pathways, with a left ear advantage in a given task interpreted as a right hemisphere advantage whereas a right ear advantage would be interpreted as a left hemisphere advantage. This was first established with dichotic listening by Doreen Kimura in 1961 (Kimura, 1961a, 1961b).
Having put these basic facts out of the way, we can say that measures of perceptual asymmetries were the easiest way to assess cerebral functions when the reviews cited earlier appeared. Some researchers were quick to note that the (presumed) areas where sex differences occurred (verbal and spatial) were believed to be processed in a different cerebral hemisphere (left or right, respectively). This led to two opposing hypotheses: Levy (1971) figured that women would be less lateralized than men as they presumably used more of their brain for verbal functions, thereby crowding the nonverbal hemisphere. Clearly lateralized functioning in males was presumed to allow good spatial skills, but lower verbal skills than women. Interestingly, Buffery and Gray (1972) came to the opposite conclusion. Although it is difficult to do justice to the reasoning followed by Buffery and Gray in a short blog post (see Voyer, 1996 for elaborations), essentially, it boils down to stating that spatial crowding occurred in men’s left and right hemisphere. Either way, although simplified here, these models both reflect a form of crowding hypothesis.
How did these models fare in relation to existing data? McGlone (1980) published possibly the most detailed literature review at the time and her findings sided with Levy. However, McGlone’s approach could not quantify the difference. When I started doing research on cognitive sex differences (in the early 1980s), the debate was still on concerning the crowding hypothesis. Therefore, examining sex differences in the brain with the methods available back then to account for cognitive sex differences was a logical way to look at theories. This is how I ended up with two major research interests: Cognitive sex differences and perceptual asymmetries.
This chain of events explains my apparently disparate research interests. In my next post, I will get into the existing evidence relevant to sex differences in the brain that examines functional and anatomical asymmetries.
References
Broverman, D. M., Klaiber, E. L., Kobayashi, Y., Vogel, W. (1968). Roles of activation and inhibition in sex differences in cognitive abilities. Psychological Review, 75, 23–50.
Buffery, A.W.H., & Gray, J.A. (1972). Sex differences in the development of spatial and linguistic skills. In C. Ounsted & D.C. Taylor (Eds.), Gender differences: Their ontogeny and significance (pp.123-157).Baltimore: Williams & Wilkins.
Garai, J.E., & Scheinfeld, A. (1968). Sex differences in mental and behavioral traits. Genetic Psychology Monographs, 77, 169±299.
Kimura, D. (1961a). Cerebral dominance and the perception of verbal stimuli. Canadian Journal of Psychology, 15, 166–171.
Kimura, D. (1961b). Some effects of temporal lobe damage on auditory perception. Canadian Journal of Psychology, 15, 156–165
Levy, J. (1971). Lateral specialization of the human brain: behavioral manifestations and possible evolutionary basis. In J.A. Kiger, Jr. (Ed.), The biology of behavior (pp.159-180). Corvallis: Oregon State University Press.
McGlone, J. (1980). Sex differences in human brain asymmetry: A critical survey. The Behavioral and Brain Sciences, 3, 215-263.
Voyer, D. (1996). On the magnitude of laterality effects and sex differences in functional lateralities. Laterality, 1, 51-83.
